Field of the Invention
The present invention relates to an onium salt which can be used for preparing a chemically amplified positive resist composition to be used for processing of a semiconductor or a photomask blank, etc., a chemically amplified positive resist composition using the same and a resist patterning process using the composition.
Description of the Related Art
In recent years, as an integrated circuit progresses toward a higher integration, a finer circuit pattern profile has been required. Among these, a chemically amplified resist using an acid as a catalyst has been exclusively used in the patterning process of the size of 0.2 μm or less. Also, high-energy beam such as ultraviolet beam, far ultraviolet beam, electron beam, etc., has been used as a light source at this time, in particular, electron beam lithography which has been utilized as an ultrafine processing technology is indispensable as a processing method of a photomask blank used for producing a photomask for a semiconductor manufacturing.
A polymer containing a large amount of aromatic skeletons having an acidic side chain, for example, a polyhydroxystyrene, has been utilized as a resist composition for KrF excimer laser, but it shows remarkable absorption in wavelength of near 200 nm of the light, so that it has been never used as a resist composition for ArF excimer laser. However, it is an important material in the point that high etching resistance can be obtained as a resist composition for electron beam or a resist composition for extreme ultraviolet (EUV), which is a useful technology for forming a smaller pattern than the processing limit by ArF excimer laser.
In general, as a base polymer of a positive resist composition for electron beam or a resist composition for EUV, mainly used is a material in which an acid-labile protective group which protects the acidic functional group of the phenol side chain contained in the base polymer is deprotected by an acid generated from a photoacid generator by irradiating high-energy beam as a catalyst whereby it is solubilized in an alkaline developer. As the above-mentioned acid-labile protective group, a tertiary alkyl group, a t-butoxycarbonyl group, an acetal group, etc., have been mainly used. Here, when a protective group such as an acetal group which requires a relatively small activation energy necessary for deprotection is used, there is a merit that a high sensitivity resist film can be obtained, but if suppression of a generating acid diffusion is insufficient, the deprotection reaction occurs even in the unexposed area of the resist film, whereby there are problems that deterioration of line edge roughness (LER) or lowering in in-plane uniformity of a pattern line width (CDU) is generated.
Various improvements by selection or combination of a material(s) to be used for the resist composition, or change in the processing conditions, etc., have been investigated for controlling the above-mentioned resist sensitivity or pattern profile. As one of the points necessary for the improvement, there is a problem of diffusion of an acid which causes serious influence on resolution of a chemically amplified resist. The problem of the acid diffusion causes serious influence on sensitivity and resolution not only in the processing of a photomask but also in a general resist composition so that many investigations have been done.
An acid diffusion controlling agent is a material to suppress acid diffusion, and is in fact an essential component to improve performances of the resist, in particular resolution. Various investigations have been done on the acid diffusion controlling agent, and amines (Patent Document 1) or weak acid onium salts have generally been used. As an example of the weak acid onium salt, in Patent Document 2, it has been disclosed that good resist pattern can be formed without forming T-top, difference in line widths between isolated pattern and dense pattern, and standing wave by adding a triphenylsulfonium=acetate. Also, in Patent Document 3, it has been stated that sensitivity, resolution and exposure margin have been improved by the addition of a sulfonic acid ammonium salt or a carboxylic acid ammonium salt. Further, in Patent Document 4, it has been stated that a resist composition for KrF and electron beam comprising a combination containing a photoacid generator which generates a fluorine-containing carboxylic acid is excellent in resolution, and process admissibility such as exposure margin, depth of focus, etc., have been improved. Moreover, in Patent Document 5, it has been also stated that a resist composition for F2 laser light comprising a combination containing a photoacid generator which generates a fluorine-containing carboxylic acid is excellent in line edge roughness (LER), and a problem of footing profile has been overcome. The above-mentioned documents concerning the onium salt relate to a material to be used for patterning using KrF, electron beam or F2 lithography. In Patent Document 6, a positive photosensitive composition for ArF excimer laser exposure containing a carboxylic acid onium salt has been disclosed. These are to suppress the acid decomposition reaction of the acid-labile group and make the acid diffusion distance small (to control) by exchanging a strong acid (sulfonic acid) generated from the other photoacid generator by exposure with weak acid onium salt to form a weak acid and a strong acid=onium salt so that a strong acid (sulfonic acid) having high acidity is exchanged to a weak acid (carboxylic acid), and thus, it apparently acts as an acid diffusion controlling agent.
However, when patterning is carried out by using the above-mentioned resist composition containing a carboxylic acid onium salt or a fluorocarboxylic acid onium salt, it involves the problem that LER is large, so that development of an acid diffusion controlling agent which can reduce LER has been desired.